This coil current flows in a coil assembly having an inductance L and forming part of a magnetic system which contains cores and/or pole pieces. In the first half of a cycle, the coil current is positive and has a constant first final value, and in the second half of the cycle, as a result of a changeover, it is negative and has a constant second final value equal in magnitude to the first final value.
The coil assembly is, for example, a single coil if the electromagnetic flow sensor serves as a flow probe, cf. U.S. Pat. No. 3,529,591, or consists of two coil halves disposed at diametrically-opposed positions on a measuring tube through which flows a fluid whose volumetric flow rate is to be measured.
U.S. Pat. No. 4,410,926 discloses a circuit arrangement for generating such a coil current. This arrangement comprises:
a bridge circuit in the form of an H network having PA1 the aforementioned coil assembly, which lies in the first bridge diagonal; PA1 a controlled current source having an output for its current; PA1 a resistor which PA1 a diode inserted between the output of the current source and the series circuit, the forward direction of this diode being equal to the direction of the current of the current source; and PA1 a capacitor of capacitance C connected in parallel with the series circuit, this capacitor and the aforementioned inductance L forming a resonant circuit whose action is such that after each reversal of the coil current, PA1 a resistor which PA1 a first capacitor of capacitance C.sub.1 connected between the second output of the first switching transistor and ground; and PA1 a second capacitor of capacitance C.sub.2 connected between the second output of the second switching transistor and ground, PA1 which forms part of a magnetic system contained in an electromagnetic flow sensor and producing a magnetic field, PA1 which comprises a core and/or a pole piece, and PA1 which has an inductance L, PA1 said coil current being positive and having a constant first final current value in the first half of a cycle, and being negative and having a constant second final current value equal in magnitude to the first final current value in the second half of said cycle; and PA1 said coil current being generated by a circuit arrangement comprising: PA1 on the one hand, to maintain the final voltage value constant for forming the first and second final current values, and, PA1 on the other hand, to compensate for the effect of eddy currents, which are induced in the cores and/or the pole pieces during the rise of the coil current and which delay the leading edge of the magnetic field with respect to that of the coil current, by PA1 which forms part of a magnetic system contained in an electromagnetic flow sensor and producing a magnetic field, PA1 which comprises a core and/or a pole piece, and PA1 which has an inductance L, PA1 said coil current being positive and having a constant first final current value in the first half of a cycle, and being negative and having a constant second final current value equal in magnitude to the first final value in the second half of said cycle; and PA1 said coil current being generated by a circuit arrangement comprising: PA1 on the one hand, to maintain the positive and negative final voltage values constant for forming the first and second final current values, respectively, and PA1 on the other hand, to compensate for the effect of eddy currents which are induced in the cores and/or the pole pieces during the rise of the coil current and which delay the leading edge of the magnetic field with respect to that of the coil current, by PA1 a first terminal of the resistor is connected to the voltage output of the voltage source, PA1 a second terminal of the resistor is connected to the junction point of the first and second transistors of the H network via a diode whose forward direction is equal to the direction of the coil current, while the junction point of the second and fourth transistors is connected to ground, and PA1 the H network is bypassed by a capacitor of capacitance C, PA1 the coil assembly is connected to ground; PA1 the resistor is connected between the junction point of the first and second switching transistors and the coil assembly; PA1 a second terminal of the first switching transistor is connected PA1 a second terminal of the second switching transistor is connected
a first bridge arm formed by the controlled current path of a first transistor, PA2 a second bridge arm formed by the controlled current path of a second transistor, PA2 a third bridge arm formed by the controlled current path of a third transistor, and PA2 a fourth bridge arm formed by the controlled current path of a fourth transistor, PA2 a first bridge diagonal between the second transistor, which is connected to the first transistor, and the fourth transistor, which is connected to the third transistor, and PA2 a second bridge diagonal between the third transistor, which is connected to the first transistor, and the fourth transistor, which is connected to the second transistor, wherein PA2 has one end connected to ground, PA2 is connected to the H network so as to form a series circuit, and PA2 is traversed by the coil current; PA2 a resonant rise of the voltage across the H network occurs and, PA2 during a rise of the coil current at the beginning of each of the aforementioned half-cycles, the coil current has a steeper leading edge than if the resonant circuit were not present. PA2 has one end connected to ground and is connected to the coil assembly so as to form a series circuit which is traversed by the coil current, PA2 a first switching transistor having a first terminal of its controlled current path connected to a second terminal of the series circuit and having a second terminal of its controlled current path connected to a first voltage output of a controlled current source, which first voltage output delivers a positive voltage appearing across the series circuit, and PA2 a second switching transistor having a first terminal of its controlled current path connected to the second terminal of the series circuit and having a second terminal of its controlled current path connected to a second voltage output of the controlled current source, which second voltage output determines a negative voltage appearing across the series circuit; PA2 said first and second capacitors and the aforementioned inductance L forming resonant circuits whose action is such that after each reversal of the coil current, PA2 a bridge circuit in the form of an H network having PA2 the coil assembly, which lies in the first bridge diagonal; PA2 a resistor connected to the H network so as to form a series circuit, a first terminal of which is connected to ground and which is traversed by the coil current; and PA2 a controlled voltage source which PA2 influencing in each half-cycle the rise time of the coil current and the magnitude of the final voltage value in such a manner that after the coil current has reached a maximum, no further rise of the coil current will occur, so that the magnetic field will reach a constant final value corresponding to the constant final value of the coil current already when the coil current reaches its maximum, PA2 with the waveform of the voltage drop across the resistor during a half-cycle after the maximum of the coil current until the attainment of the final current value being sampled at least three times in succession to form a correction quantity for the voltage across the H network in the next half-cycle. PA2 a controlled voltage source having a first voltage ouput and a second voltage output, and PA2 a T network comprising: PA2 influencing in each half-cycle the rise time of the coil current and the magnitude of the positive and negative final voltage values in such a manner that after the coil current has reached a maximum, no further rise of the coil current will occur, so that the magnetic field will reach a constant final value corresponding to the constant final value of the coil current already when the coil current reaches the maximum, PA2 with the waveform of the voltage drop across the resistor during a half-cycle after the maximum of the coil current until the attainment of the final current value being sampled at least three times in succession to form a correction quantity for the voltage across the T network in the next half-cycle. PA2 said capacitor and the inductance L forming a resonant circuit whose action is such that PA2 via the cathode-anode path of a first diode to the first voltage output of the controlled voltage source and PA2 via a first capacitor of capacitance C.sub.1 to ground; and PA2 via the anode-cathode path of a second diode to the second voltage output of the controlled voltage source and PA2 via a second capacitor of capacitance C.sub.2 to ground,
either the first and fourth transistors PA3 or the second and third transistors are simultaneously on, whereby the coil current alternately reverses its direction; PA3 a resonant rise of the voltage across the T network occurs and, PA3 during a rise of the coil current at the beginning of each of the aforementioned half-cycles, the coil current has a steeper leading edge than if the resonant circuit were not present. PA3 a first bridge arm formed by the controlled current path of a first transistor, PA3 a second bridge arm formed by the controlled current path of a second transistor, PA3 a third bridge arm formed by the controlled current path of a third transistor, and PA3 a fourth bridge arm formed by the controlled current path of a fourth transistor, PA3 a first bridge diagonal between the second transistor, which is connected to the first transistor, and the fourth transistor, which is connected to the third transistor, and PA3 a second bridge diagonal between the third transistor, which is connected to the first transistor, and the fourth transistor, which is connected to the second transistor, wherein PA4 either the first and fourth transistors PA4 or the second and third transistors are simultaneously on; PA3 has a voltage output and PA3 determines a voltage developed across the series circuit; PA4 said voltage having in each half-cycle an initial voltage value during a rise time of the coil current--as a first subcycle--which is higher than a final voltage value during a second subcycle representing the remainder of the half-cycle; PA3 a resistor connected to the coil assembly so as to form a series circuit, a first terminal of which is connected to ground and which is traversed by the coil current, PA3 a first switching transistor having a first terminal of its controlled current path connected to a second terminal of the series circuit and having a second terminal of its controlled current path connected to the first voltage output of a controlled current source, which first voltage output delivers a positive voltage appearing across the series circuit, and PA3 a second switching transistor having a first terminal of its controlled current path connected to the second terminal of the series circuit and having a second terminal of its controlled current path connected to the second voltage output of the controlled current source, which second voltage output delivers a negative voltage appearing across the series circuit, PA4 said positive and negative voltages having in each half-cycle a positive initial voltage value and a negative initial voltage value, respectively, during a rise time of the coil current--as a first subcycle--which are higher than a positive final voltage value during a second subcycle as the remainder of the half-cycle and lower than a negative final voltage value during said second subcycle, respectively; said method using the voltage drop across the resistor, PA3 a resonant rise of the voltage across the H network occurs, and PA3 that during its rise time, the coil current has a steeper leading edge than if the resonant circuit were not present. PA3 said first and second capacitors and the inductance L forming respective resonant circuits whose action is such that PA4 a resonant rise of the voltage across the series circuit occurs, and PA4 that during its rise time, the coil current has a steeper leading edge than if the resonant circuit were not present.
U.S. Pat. No. 4,410,926 also discloses a circuit arrangement for generating a coil current which comprises a T network comprising:
U.S. Pat. No. 4,204,240 discloses a circuit arrangement with a voltage source for generating the coil current of an electromagnetic flow sensor which delivers a voltage having in each half-cycle an initial voltage value during a rise time of the coil current--as a first subcycle--which is higher than a final voltage value during a second subcycle representing the remainder of the half-cycle.
The cores and/or pole pieces of the magnetic system are generally made of soft magnetic material, but magnetic systems with ferromagnetic cores have also been described.
In both types of magnetic systems, the changeover and the rise of the coil current cause eddy currents to be induced in the magnetic system which prevent the rise of the magnetic field from exactly following the rise of the coil current, as would be the case without cores and/or pole pieces. By contrast, the rise of the magnetic field is delayed with respect to that of the coil current and levelled off. This disadvantageous effect of the eddy currents also occurs at and despite of the aforementioned resonant rise.
The effect of the eddy currents can be illustrated by an equivalent circuit diagram in which the (pure) inductance L is shunted by an eddy-current source whose current adds to the current in the (pure) inductance L to form a total coil current, which also flows through the resistor. Thus, the voltage drop across the resistor is only a measure of the total coil current, but not a measure of the (pure) coil current. This, however, is necessary for maintaining the coil current at a constant level.
In U.S. Pat. No. 4,784,000, a solution to this this overall problem is indicated in FIG. 6. It is assumed, however, that in the uncontrolled condition, the coil current during each total half-cycle is not constant, and sampling takes places twice in succession in a range in which the coil current is constant in the controlled condition. However, the two sampling operations in this range have turned out to be insufficient.
The invention serves to provide an improved solution to this overall problem; therefore, it is to provide a method which completely eliminates the detrimental effect of the eddy currents on the rise and the rise time of the magnetic field, so that the magnetic field reaches its constant final value already when the coil current assumes its maximum value.